Flexible sheet-like magnetic discs having a nominal standardized size of about 3.5 inches are widely used in a variety of data storage/retrieval systems. For example, cassettes which include flexible magnetic discs are used in conjunction with personal computers so as to load data and/or programs into the central processing unit of the computer, as well as to store data in an off-site location in a more convenient manner.
When placed into service in a magnetic recording/reproducing apparatus, the magnetic disc (which is accommodated for rotational movement within the interior of the cassette case) is caused to spin in the desired direction and at the desired rotational velocity relative to a magnetic read/write head by means of a motor-driven spindle coacting with an aperture in the central hub of the magnetic disc. The spindle, moreover, serves to center the magnetic disc relative to the magnetic read/write head so that accurate placement and retrieval of data onto and from the disc will ensue.
The recording/reproducing apparatus will also usually include an electromagnet (usually provided integrally as part of a drive carriage which also includes the motor-driven spindle) which attracts the central hub of the data disc when in service so as to ensure positive contact with the spindle. The central hub of conventional flexible data discs must therefore necessarily be formed of a material which exhibits ferromagnetic properties--i.e., behaves ferromagnetically when exposed to a magnetizing force. Permanent magnetic material (i.e., magnetically "hard" material) would, however, detrimentally affect the magnetic data storage functions of the magnetic media associated with the data disc. For this reason, the data disc core is conventionally formed of a magnetically "soft" stainless steel.
Use of stainless steel as the central hub of a data disc, however, presents its own problems. For example, the stainless steel hub and the magnetic media (typically a circular flexible sheet of polyester coated with a magnetic film) exhibit different thermal expansion properties. There exists the possibility, therefore, that the adhesive bonding between the stainless steel hub and magnetic media may become loose due to repeated thermal expansion/contraction cycles thereby rendering the data disc unusable.
In addition, the stainless steel stock must be subjected to a number of metal-forming operations in order to achieve the necessary geometry and dimensional attributes required of a data disc hub. Exposure to repeated metal-forming operations raises the likelihood that a relatively large percentage of stainless steel central hubs will be rejected by quality control standards. That is, since each fabrication step carries with it the risk that the hub will not be formed to design standards for that particular metal-forming operation, an increase in the number of fabrication steps should likewise result in an increase in the number of rejected parts.
Recently, it has been proposed in U.S. Pat. No. 4,941,066 to Swinburne et al to form a central data disc hub by incorporating a flat stainless steel insert with a molded plastics core. The formation of the metal insert by stamping from flat metal is said to be much easier as compared to forming a conventional hub from stainless steel. The Swinburne et al '066 patent also mentions an alternative to the core/insert arrangement for their central hub. Specifically, at column 4, lines 35-43, Swinburne et al mention the possibility of forming the center core from a magnetized plastic, in which case the flat metal insert is not needed. No suggestion is made, however, of the manner in which plastics generally may be magnetized. Nor is there any suggestion that magnetized polyacetal could be employed.